Beamforming multibeam sonars include an acoustic array of many receiving channels. Each receiving channel includes a transducer, such as a microphone and/or hydrophone, and signal processing electronics. Signals from all channels are processed in order to obtain the desired directional response (beam) of the array. In fact by a proper signal processing it is possible to generate many beams, which will cover the region of interest. The directional response is sometimes also referred to as the beam pattern.
Different techniques for calibration of beamforming multibeam sonars have previously been described. For example, it has been proposed to calibrate the beamformed response (i.e. the overall directional response of the array when applying beamforming) of a multibeam sonar with respect to amplitude as a function of direction/angle. Such a calibration has been proposed using so-called “standard targets,” i.e. standardised metal spheres having known acoustic reflection characteristics.
U.S. Pat. No. 4,468,760 describes a calibration system that can measure and display the real directional response of a hydrophone, i.e. the array output amplitude as a function of incident angle. The system can further measure and display the complex sensitivity of a hydrophone, i.e. the amplitude and phase as a function of frequency.
The article “Measurement of multibeam sonar directivity patterns”, by Dezhang Chu et al. Proc. of OCEANS CONFERENCE, 2002, VOL 3, pages 1411-1414, describes a calibration experiment with a multibeam sonar system.
The article “Method for Large Sonar Calibration and Backscattering Strength Estimation” by Pawel Pocwiardowski, George Yufit, Eric Maillard, and Peter Eriksen, in Proceedings of Oceans 2006, Boston, September 2006 describes a calibration method based on a separate calibration of the acoustic and electrical components.
Another approach is disclosed in U.S. Pat. No. 5,552,791 which describes a calibration based on a laser calibration signal sent into the signal pass such that the calibration signal replaces the signal otherwise measured by the sensors during normal operation. Hence, this prior art calibration technique is concerned with the spatial distribution of the sensors and the resulting phase difference between them due to different signal path lengths within the system.
Embodiments and other aspects described herein seek to improve the suppression of sidelobes of a sonar apparatus such as a beamforming sonar by performing an improved calibration of the obtained acoustic echo signals.